Presynaptic mechanisms controlling excitatory and inhibitory synaptic transmission in the cerebral cortex have critical roles in normal information processing and also may contribute to the pathophysiology of a variety of brain disorders such as cognitive decline and epilepsy. The proposed experiments focus primarily on inhibitory synaptic transmission mediated by gamma amino butyric acid (GABA)-containing fast-spiking (FS) inhibitory interneurons and its modulation by activities of sodium-potassium ATPase (NKA, the sodium pump). FS interneurons mediate the most common and strongest form of inhibition and are known to be vulnerable to injury. Preliminary data show that immunoreactivity for ?3KA, the main pump isoform in interneurons, is decreased in axonal terminals of FS cells in partially isolated (undercut or UC) neocortex. We will test the hypothesis that FS to excitatory cell GABAergic transmission will be compromised by decreases in ?3KA activity, whether it occurs in partially isolated epileptogenic cortex, via treatment with cardiotonic steroids, or in epileptic mice with genetically altered NKA alpha3 subunit. Specific aims are to 1) assess effects of cardiotonic steroids or cortical injury on pump current and the dynamics of FS to excitatory cell inhibition; 2) study functional effects of enhancing ?3KA expression in injured or Myshkin mouse cortex with a small molecule partial TrkB agonist; and 3) test the hypothesis that properties of FS GABAergic neurons and transmission from FS cells will be selectively altered in Myshkin mice with a genetically induced epilepsy due to mutations in the ?3 pump subunit. Techniques will include paired recordings in cortical slices to obtain unitary (u) inhibitry postsynaptic currents (IPSCs) generated selectively by FS cells; optogenetic approaches in parvalbumin/ChR2 mice to selectively activate FS cells; and both laser scanning photostimulation and multiple simultaneous extracellular recordings to monitor effects network effects. The partial cortical isolation model will be used to provide chronically injured, epileptogenic slices and assess changes in pump function that might contribute to hyperexcitability. The goals are to identify pump activities that influence GABAergic transmission, abnormalities induced by injury or genetic alterations in pump function that may contribute to hyperexcitability and epileptogenesis; and test whether enhancing NKA expression will rescue compromised FS cell-mediated inhibition in the UC cortex.